1. Field of the Invention
The invention relates to the field of reusable fuses derived from positive temperature coefficient (PTC) resistive sections arranged for protecting electrical power distribution systems and loads coupled thereto. In particular, the invention concerns a sandwich arrangement of thermally engaged current limiting PTC sections in series with a load such as a motor and controlling a load actuator such as a motor starter, whereby the current limiting polymer is protected from excessive power dissipation.
2. Prior Art
Positive temperature coefficient (PTC) resistive elements are known and are known to be used as reusable fuses. A PTC resistive element exhibits a relatively low resistance to a flow of electrical current when the current is below a threshold value. Current above the threshold value flowing through the PTC causes resistive heating of the PTC element. A rise in internal temperature of a PTC element to above a transition temperature causes the PTC element to transition into a state of high resistance, thereby limiting current flow through the PTC element and the circuit containing it.
As stated in PTC Application Notes, Keystone Carbon Company Bulletin T-929, P.37, "[t]he dramatic rise in resistance of a PTC at the transition temperature makes it an ideal candidate for current limiting applications. For currents below the limiting current (1.sub.L), the power being generated in the unit is insufficient to heat the PTC to its transition temperature. However, when abnormally high-fault currents flow, the resistance of the PTC increases at such a rapid rate that any further increase in power dissipation results in a reduction in current."
Examples of PTC resistive devices include silicon carbide, tungsten, polycrystalline ceramic barium titanate or barium and strontium titanate and current limiting conductive polymers.
Current limiting conductive polymers are known in the art to be useful for limiting electrical current. For example, Raychem Corporation manufactures and markets a current limiting polymer under the trademark Polyswitch.TM.. Current limiting polymers having PTC characteristics are disclosed in U.S. Pat. No. 4,545,926; 4,560,498 and 4,775,778, all owned by Raychem Corporation. Current limiting polymers typically comprise cross-linked polyethylene, heavily doped with carbon. PTC's typically have a low electrical resistance when conducting current below a threshold value, i.e., when the PTC is relatively cool. When current flowing through the PTC exceeds the threshold, resistive heating produces a rise in the internal temperature of the PTC, causing a reduction in conductivity, i.e, an increase in electrical resistance. The power dissipated in the PTC is proportional to the resistance multiplied by the square of the current, therefore an increase in resistance leads to a further increase of resistance. The change in resistance thus is quite rapid. Typically, the increase in resistance is virtually a step function once the magnitude of the current (and the resulting internal temperature of the polymer) surpasses the threshold value.
The change in resistance of a PTC upon passing the threshold is quite large. For example, the resistance of a current limiting polymer upon passing the threshold may increase by a factor of 1,000 to 4,000. Assuming the PTC is in a power system in series with a load, the increase in resistance of the PTC increases the total load resistance seen by the power line and substantially reduces the current. However, the increase in resistance of the PTC produces a corresponding increase in the voltage drop across the PTC and a decrease in the voltage drop across the load. Thus, a larger portion of the power from the line is dissipated in the PTC as heat, as opposed to being dissipated by the load. Depending on the application (i.e., the line voltage and the load resistance), the voltage drop across a PTC which has transitioned to its high resistance state could be substantial, and could result in destruction of the PTC. This is especially true when a conductive polymer is used as the PTC. Furthermore, PTC's are known to exhibit negative temperature coefficient (NTC) resistance characteristics if the internal temperature of the PTC goes much beyond the threshold level. If heated to the NTC level, the resistance of the PTC decreases.
A relatively large PTC section can be used to improve the ability of the PTC to dissipate heat. However, generally the PTC material is relatively expensive, and this approach may require the use of a large amount of PTC material to provide adequate power dissipating capacity. The extra material will increase the bulk of the PTC and decrease the ease of installing the PTC in the circuit for use as a reusable fuse.
Placing a PTC in thermal communication with a heat producing component is known in the art. Such an arrangement is disclosed in U.S. Pat. Nos. 4,780,598 and 5,064,997, both owned by Raychem Corporation. As disclosed in U.S. Pat. Nos. 4,780,598 and 5,064,997, the heat producing component is a voltage-dependent resistor. The voltage-dependent resistor and the PTC are electrically coupled in a series circuit with components to be protected from excessive current flow. The heat producing component radiates heat to the PTC to accelerate its transition into a state of high resistance to protect the other circuit components. No provision is made to decouple power from the PTC or heat producing component once the transition occurs. The PTC, therefore, is subject to destruction from excessive power dissipation. Furthermore, neither of U.S. Pat. Nos. 4,780,598 or 5,064,997 discloses disposing PTC elements coupled in separate but related circuits in an abutting, thermally communicating relationship wherein excessive heat generating current in one circuit produces a reaction in the other circuit.
There is a need, therefore, for a current limiting device that takes advantage of the beneficial aspects of a current limiting PTC for protecting a line or load from overcurrent conditions with a reduced risk that the PTC will be destroyed by heat dissipation from the increase in voltage drop across the PTC that is inherent in its operation. Advantageously, this should be accomplished without requiring a substantial increase in the amount of PTC used in the circuit, to minimize the expense of the protective device.